A comparison of iterative minimum norm estimation and current dipole estimation for magnetic field measurements from small animals

We developed a multi-channel high-resolution SQUID magnetometer for measuri ng biomagnetic fields produced by small animals, The diameter of the pickup coils in our system is 5 mm, i.e., smaller than typical systems. 12 magnet ic fields on a 7.5 mm grid were measured with better spatial resolution tha n typical systems, We measured the magnetic field response to auditory stim uli in rat brains in a magnetically shielded room. Eight male rats were ane sthetized intraperitoneally with Urethane (1 g/1 kg) and stimuli consisting of 3 Hz tone burst sounds were introduced to the left ears. Averaging of t he magnetic fields was performed for over 3000 auditory stimuli. The locali zation of current sources was calculated by two methods, the iterative mini mum norm estimation (MNE) and the current dipole model (CDM), Current sourc es were localized in the auditory area of the right hemisphere in the rat's cortex by iterative MNE, Estimated sources by CDM were localized more deep ly than estimated current sources by iterative MNE, The correlation coeffic ient between the measured magnetic fields and the reconstructed magnetic fi elds in current sources was higher for iterative MNE than CDM, Iterative MN E is useful for source localization of auditory cortical activities in rats , Our system is a useful and noninvasive tool for brain research in small a nimals.